1. Field of the Invention
The present invention relates to an amorphous carbon material used for a negative electrode of a lithium ion secondary battery. More specifically, the present invention relates to an amorphous carbon material used for a negative electrode of a lithium ion secondary battery with suppressed capacity degradation and high durability, a negative electrode comprising the amorphous carbon material and a lithium ion secondary battery comprising the negative electrode.
2. Description of the Related Art
A lithium ion secondary battery is light-weighted and has high input/output characteristics compared with conventional secondary batteries such as a nickel-cadmium battery, a nickel-metal hydride battery and a lead battery. Accordingly, the lithium ion secondary battery has been expected in recent years as power supply for electric vehicles and hybrid vehicles. Typically, this kind of battery comprises a lithium-containing positive electrode enabling reversible intercalation of lithium and a negative electrode of a carbon material, the positive and negative electrodes being opposed to each other via a non-aqueous electrolyte. This kind of battery is assembled in a discharge state, and do not become a dischargeable state without be charged. The following describes the charge and discharge reactions by way of a typical conventional embodiment comprising a lithium cobaltate (LiCoO2) as a positive electrode, a carbon material as a negative electrode and a non-aqueous electrolyte solution containing a lithium salt as an electrolyte.
During charge of a first cycle, lithium comprised in the positive electrode is firstly released to the electrolyte solution (Formula 1 below), so that the positive electrode potential shifts to a noble direction. At the negative electrode, lithium released from the positive electrode is absorbed by the carbon material (Formula 2 below), so that the negative electrode potential shifts to a less noble direction. Typically, when a difference between the positive electrode potential and the negative electrode potential, i.e., a battery voltage reaches a predetermined value, the charge is terminated. This value is referred to as a charge termination voltage. Then, during discharging, lithium absorbed by the negative electrode is released, so that the negative electrode potential shifts to a noble direction, and the lithium is absorbed again by the positive electrode, so that the positive electrode potential shifts to a less noble direction. Similarly to the charging, discharge also is terminated when a difference between the positive electrode potential and the negative electrode potential, i.e., a battery voltage reaches a predetermined value. That value is called a discharge termination voltage. The whole reaction formula of such charge and discharge is shown in the Formula 3 below. In the following second cycle or later, the charge and discharge reactions (cycles) progress as lithium moves between the positive electrode and the negative electrode.

In general, a carbon material used for a negative electrode material of a lithium ion secondary battery is broadly divided into a graphite material and an amorphous material. The amorphous carbon material has an advantage of having higher output characteristics than the graphite carbon material so that the amorphous carbon material is used in a lithium ion secondary battery for vehicles and electric power storage infrastructure. The higher output characteristics are required, for example, for vehicles at the time of start from a stopped state and for electric power storage infrastructure at the time of leveling a sudden load fluctuation.
As stated above, this kind of battery has been examined actively for the use as electric storage devices for vehicles, industry, and electric power supply infrastructure in recent years. When the battery is used for these applications, these applications require extremely high degree of reliability compared with the applications for mobile phones or laptop computers. The term “reliability” is a property related to product life, referring to a property of hardly changing (hardly deteriorating) in charge/discharge capacity and internal resistance when the battery is subjected to repeated charge/discharge cycles, when it is stored while being charged to be a predetermined voltage, or when it is charged (floating charged) continuously at a constant voltage.
Incidentally, it is generally known that a lithium-ion secondary battery conventionally used for mobile phones and laptops has life characteristics greatly depending on the negative electrode material as well. One of the reasons is that it is impossible in principle to make the charge/discharge efficiency identical between the positive electrode reaction (Formula 1) and the negative electrode reaction (Formula 2). The charge/discharge efficiency is lower at the negative electrode. The charge/discharge efficiency refers to a ratio of dischargeable electric capacity to the electric capacity consumed by charging. The following is a detailed description on reaction mechanism to degrade the life characteristics due to such low charge/discharge efficiency of the negative electrode reaction.
During charging, lithium is released from the positive electrode (Formula 1) and is absorbed by the negative electrode (Formula 2) as stated above, where the reactions at the positive electrode and the negative electrode consume the same amount of electric capacity during charging. The charge/discharge efficiency, however, is lower at the negative electrode, so that during the subsequent discharging reaction, the discharge is terminated in the state where the amount of lithium released from the negative electrode is less than the amount of lithium that can be absorbed on the positive electrode side, i.e., the amount of lithium that has been absorbed before the charge on the positive electrode side. This is because a part of the electric capacity consumed by charge at the negative electrode is consumed by a side and/or competitive reaction, but not by the reaction of absorbing lithium, i.e., the absorbing reaction as dischargeable capacity.
As a result of such charge/discharge reactions, the positive electrode potential at the discharge termination state shifts to a nobler direction than the original potential before the charge/discharge, and the negative electrode potential also shifts to a nobler direction than the original potential before the charge/discharge. This results from the following reason. All of lithium which has been released during the charge of the positive electrode, cannot be absorbed (not return to) during discharging. Thus, during discharging when the potential shifted to a noble direction during charging is shifted to a less noble direction, the potential cannot return to the original positive electrode potential by an amount corresponding to a difference in charge/discharge efficiency between the positive/negative electrodes. Then discharge is terminated at a nobler potential than the original positive electrode potential. As stated above, since the discharge of a lithium ion secondary battery ends when the battery voltage (i.e., a difference between the positive electrode potential and the negative electrode potential) reaches a predetermined value (discharge termination voltage), a nobler potential at the positive electrode at the time of discharge termination means that the negative electrode potential shifts accordingly to a noble direction.
As stated above, during the repeated charge/discharge cycles, this kind of battery generates a problem of a reduction in capacity obtainable within a predetermined voltage range (within the range between the discharge termination voltage and the charge termination voltage) owing to a change of an operation range of the capacity of the positive/negative electrodes. Such reaction mechanism of the capacity degradation has been also reported in academic conferences and others (e.g., Proceedings of the 48th Battery Symposium in Japan, 1A11 (Nov. 13, 2007) and Proceedings of the 76th Meeting of the Electrochemical Society of Japan, 1P29 (Mar. 26, 2009)).
Electron Spin Resonance (ESR) studies on carbon have been reported (Tanso 1966, No. 47, 30 to 34; Tanso 1967, No. 50, 20 to 25; and Tanso 1996, No. 175, 249 to 256).